A. Field of the Invention
The invention relates to the field of metal forming, and particularly to a method of making metallic sandwich structures by superplastic forming and diffusion bonding.
B. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example some titanium alloys, such as Ti-6Al-4V, have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult or, in some instances, not possible. A significant breakthrough in superplastic forming was made by Hamilton, et al., disclosed in U.S. Pat. No. 3,934,441, Controlled Environment Superplastic Forming, incorporated into this specification herewith by reference. Simplified, the process involves placing a worksheet or blank over a cavity in a chamber. The blank is heated to a temperature where it exhibits superplastic characteristics after which pressure is applied to the blank, causing it to stretch and form into the cavity.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, Method of Making Metallic Sandwich Structures, by Hamilton, et al., and is herein also incorporated by reference.
Basically, the Hamilton, et al. method for making metallic sandwich structure involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are coated in selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a die assembly. The stack is diffusion bonded together in the uncoated areas by the application of pressure, and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size, and shape of the joined areas.
This process requires a die which is typically machined from material that has high strength at the elevated temperatures involved in superplastic forming. While such dies are cost effective when used on large production runs, the cost of manufacturing such dies for use in producing prototype structures or on small production runs may be uneconomical.
It is therefore a primary object of the present invention to combine the processes of superplastic forming and diffusion bonding to form prototype on small production runs of metal sandwich structures.
It is another object of the present invention to provide a low-cost method of making prototype and limited production run metal sandwich structures.
It is yet another object of the present invention to provide a low-cost method of making dies for use in superplastic forming and diffusion bonding of structures by utilizing the lost wax investment mold process.